1. Field of the Invention
The invention relates generally to the field of digital radio signal communications. More particularly, the invention relates to measuring received signals to determine whether to transmit using diversity.
2. Description of the Related Art
Radio communications are sensitive to the environment in which the radio signals are transmitted and received. Communications can be lost or degraded when the environment is not favorable to the type of signals being transmitted. One particular environmental problem is sometimes referred to as clutter or scattering. This problem can cause multipath interference or completely prevent reception of a transmitted signal. One known way to mitigate the effects of clutter or scattering is through receive or transmit diversity. Spatial receive diversity typically uses two receive antennas a short distance from each other. The receiver will either choose the antenna that receives the strongest signal or use an equalizer to combine the signals from both antennas. There are many different equalizer designs well known in the art. Time receive diversity uses a single antenna and searches for multipath copies of a signal delayed in time. An equalizer then combines the two or more identical signals to resolve the original single signal. Digital cellular telephones are required to have time diversity receive equalizers that can resolve signals with one to four symbols of delay spread depending on the system.
Transmit diversity works much like receive diversity in reverse. Signals can be transmitted from spaced antennas or from the same antenna at two different times. Transmit diversity schemes can enhance reception significantly under certain circumstances. However, they increase the complexity of the transmitter and, in a communications system with more than two terminals, the additional transmissions increase the amount of interference and noise for the other terminals. In addition, for some environmental conditions, diversity transmission can reduce the quality of the received signal.